1. Field of the Invention
The present invention relates to a flow rate control unit particularly used in a vending machine, for example, to supply a constant amount of drinking water.
2. Prior Art
Referring to FIG. 1 which shows a general conventional flow rate control unit to which the present invention is applied, an upper opening OU and a lower opening OD are disposed at the upper and lower ends of a cylindrical housing 8 respectively, and a fluid outlet 1 is disposed on the side of the cylindrical housing 8. An adjustment screw 9 is engaged with the screw section formed on the above-mentioned upper opening OU. A pair of flanges 9A and 9B are formed on the upper and lower circumferences of the adjustment screw 9. Between the pair of the flanges 9A and 9B, an O-ring 11 is fitted in. In a step section 12 located inside the housing 8, an outer cylinder 3 is inserted, the diameter of which is slightly smaller than the inner diameter of the housing 8. A plurality of fluid passing holes 2 leading to the fluid outlet 1 are disposed around the circumference of the outer cylinder 3. In addition, an inner cylinder 7 which slides on the inner sliding surface of the outer cylinder 3 contains an elastic member 6 (coil spring) whose elastic force is normally applied to the inner cylinder 7, the upper end of which is supported by the adjustment screw 9. At the lower section of the inner cylinder 7, a fluid inflow hole 5 is disposed. Around the inner cylinder 7, a fluid storage section 13 is provided. An inlet fitting 14 equipped with a fluid inlet 4 is secured together with the above-mentioned outer cylinder 3 by using a coupling nut 15 to close the above-mentioned opening OD located at the lower section of the housing 8.
In this structure, depending on the pressure difference between the inner section S and the outer section T of the inner cylinder 7, the inner cylinder 7 moves up and down inside the outer cylinder 3 with a ring-shaped slight clearance 18 provided therebetween. The opening area of the fluid passing holes 2 changes depending on the position of the upper edge 17 of the inner cylinder 7 due to the up-and-down movement of the inner cylinder 7. Therefore, if the pressure in the outer section T is larger than that in the inner section S, the inner cylinder 7 moves up to decrease the opening area of the fluid passing holes 2. If the pressure in the outer section T is smaller than that in the inner section S, the inner cylinder 7 moves down to increase the opening area of the fluid passing holes 2. As a result, the discharge flow rate from the fluid outlet 1 is made constant at all times. The fluid stored in the storage section 13 of the inner cylinder 7 form a water film to lubricate the clearance 16. Some of the above-mentioned outer and inner cylinders 3 and 7 of such a flow rate control unit are made of ceramics, such as alumina, instead of conventionally used stainless steel, to prevent corrosion due to drinking water (Japanese Provisional Patent Publication No. 60-121371 for example). The average surface roughness Ra.mu.m (more particularly the medial line average roughness Ra.mu.m, but hereafter simply referred to as "the average surface roughness Ra.mu.m) of the sliding surfaces of these outer and inner cylinders 3 and 7 is about 0.2 .mu.m (0.8 s); the surfaces are significantly smooth. The medial line average roughness Ra.mu.m is determined as follows: When a straight line which is drawn in parallel with the average line of a roughness curve divides the areas enclosed by the straight line and the roughness curve into two sides which are equal in area, the straight line is referred to as "a medial line." The medial line average roughness Ra.mu.m indicates an uneven value (integration value) in the orthogonal two-dimensional directions based on the medial line. This roughness unit is used in Japan (JIS B0601-1982) and the United States of America as an international unit.
In the case of the above-mentioned outer and inner cylinders 3 and 7, a water film is formed between their sliding surfaces to provide a sealing effect. However, even when the above-mentioned storage section 13 for example is provided, if ceramics is used and if the roughness of the surfaces sliding with each other is too small, the circumferential surfaces stick together due to the water film, thereby being apt to cause malfunction. On the other hand, if the surfaces are too rough, resistance occurs due to scratching, eliminating a sealing effect and also being apt to cause malfunction. This fluctuates flow rate control; fluid cannot flow at a constant flow rate at all times.